CH load & Efficiency

[desktop987456]

Hi,
My viessmann heat only boiler 13kw (OV) is running at flow temp of 54.5DegC and a return temperature of 40DegC. I know it is a crude way of getting the return temperature on boiler. In the 2nd picture showing the return temperature you can spot the sensor attached to return pipe next to filter.

I have following Rads heating 4 bedrooms and 3 baths. These T50ratings were taken from Manufacturer's catalogue.

Rad	T50 Rad output kw	T 28.5 Rad output kw
Bedroom GF	1.3​	0.64​
Bedroom UP	2.08​	1.03​
Bedroom 2 Up	1.56​	0.77​
Guest Bed	1.10​	0.54​
Bath 1	0.80​	0.39​
Bath 2	0.70​	0.35​
Bath 3	0.65​	0.32​
CH Load kW	8.19​	4.04​

​

I have logged m3 consumed from gas meter running only boiler for 1 hour.
Boiler was in steady state (meaning it was on for the last 3 hours)

@1800 hrs on​	8096.04m3​
@1832 off​	8096.48m3​
@1843 on​
@1900 hrs kept running till I logged the meter​	8096.74m3​

In 1 hour of usage it ran for almost 50 minutes and consumed 0.625m3 X 10.8 = 6.75kwh. CH dial on boiler is just below 4
Flow temperature on boiler was 54.3 DegC and a return temperature of 40DegC.
I am running boiler at 54.3+40=94.3/2. Thermostat is set at 19 DegC (and is installed in the coldest area of the property; lounge which was not heated during this time) so my radiators are effectively rated at 28.5DegC, Correct?
This means my boiler is only 55% efficient, I don't understand, there must be something wrong somehere, Could someone please advise what am I doing wrong.
Flow temperature dial is set just below the delivered condition (pic 3). It used to get to 49.5-50DegC at this setting and would turn off at that temperature. Outside temperature has dropped couple of degrees due to a cold wave during the last couple of days (where I live).
Thanks
Edit : Pump is external and is at C2 setting (medium) Flomasta CP50 (6m head)

 

[MNW67]

I think most likely is that your radiators are giving out a lot more heat in practice than they are rated for. It's not clear to me how a boiler could lose 30%+ efficiency.

 

[Johntheo5]

As above.

A T50 rad running at the above conditions, assuming boiler flow/return of 54.3C/40C accurately reflect the rad(s) flow/return will emit 47.4% of its T50 rating so based on your T50 rating of 8.1kw at T50 then the rad/boiler demand should have been 8.1*0.474, 3.84kw, so, mathemaically, if you consumed 6.75kwh of natgas then the boiler efficiency is, 3.84/6.75, 56.9%.
On the other hand, if your boiler is running normally, a 40C return temp should mean at least 90% efficiency so boiler output was 0.9*6.75, 6.08kw, based on that, with the rad(s) output (running as T28.15) at 6.08kw which means their total T50 rating is 6.08/0.474, 12.83kw??.

Its relatively easy to see if your boiler is running efficiently, just stick a probe in the fluegas analysis port and measure the temperature.
boiler efficiency changes by ~ 4.5%/100C (fluegas temp). At 100C fluegas temp, the boiler efficiency (assuming 20C combustion air temp) would be ~ 86.4%, (3.6% sensible heat gas loss + 10% non condensing loss). If your boiler is only running at 56.9% efficiency then the fluegas temp must be somewhere in the region of 736C, (33.12% sensible heat loss + 10% wet gas loss) highly unlikely, your chimney would have melted.

 

[desktop987456]

radiators are giving out a lot more heat in practice

Rads are the same as last year when I had non condensing boiler. I think I may have skewed heatloss calcs a bit, not sure.

how a boiler could lose 30%+ efficiency.

Hence this post.

 

[desktop987456]

a 40C return temp should mean at least 90% efficiency so boiler output was 0.9*6.75

Correct. I have measured the return temp of boiler, collected and measured condensate which was about ~1.5 litres per hour. This tells me that boiler is condensing but when I try to calculate efficiency based on heatload they don't add up?
The boiler's turn down ratio is 1:3 so at 12.9 kw max its minimum output should be 4.3kw / 10.8 = 0.4m3/hr of gas.
I'll revisit my rad output numbers again.

 

[desktop987456]

I have also tried to change the pump speed to its lowest setting (C1) but at this flow temperature, heating in rooms couldn't get to temperature. Rads were luke warm and boiler kept on running at its minimum flame level and was on a bit longer than usual.

Once CH is upto temperature the boiler modulates (flame level turns down at 20% well before setpoint is reached). Not sure if it has anything to do with CH load?

 

[Johntheo5]

Correct. I have measured the return temp of boiler, collected and measured condensate which was about ~1.5 litres per hour. This tells me that boiler is condensing but when I try to calculate efficiency based on heatload they don't add up?
The boiler's turn down ratio is 1:3 so at 12.9 kw max its minimum output should be 4.3kw / 10.8 = 0.4m3/hr of gas.
I'll revisit my rad output numbers again.

Its almost impossible IMO to measure boiler efficiency using the rads as a accurate indication of load due to flow temperatures, different flowrates through each rad unless perfectly balanced etc.
Your best bet is definitely using the boilers own data and as I stated above, the flue gas temperature which alone will be quite enough to enable the efficiency to be determined even using the simple graph, below, even though this doesn't use the fluegas temperature, measuring the condensate flow is also very helpful and of course the natgas consumption.
At 40C return temp, the boiler is ~ 95% efficient.
Your 12.1kw boiler at full output will use 12.1/0.95, 12.74kwh.
If you look at two scenarios re condensate, one assumes the boiler is fully condensing (impossible, in practice) the other is your case above.
If fully condensing at 99.5% efficiency then the boiler will use 12.1/0.995, 12.16kwh.
The latent heat of vaporization of water is 2260kj/kg, since a kg of water is practically the same as a litre at these temperatures then latent heat is 2260kj/litre or 2260/3600, 0.628kw/litre.
The wetgas loss of natural gas varies a bit due to the methane/hydrogen content but 10% is good enough.
At full condensing and full output your boiler will have a wet gas loss/gain of 12.16*10%, 1.216kwh.
The condensate flow will be 1.216/0.628, 1.94 liters.

At 40C return and full output the boiler will use 12.74kwh.
the % condensing will ~ be 99.5-95, 4.5%
At 4.5% condensing and full output your boiler will have a wet gas loss of 12.74*4.5%, 0.573kwh.
The condensate flow will be 0.573/0.628, 0.91 liters.

I know you got a condensate flow of 1.5L at a much lower output, can't really comment on that, I think some boiler manufacturers publish the condensate flow in their MIs, it would be interesting if one could be found to see how it compares with those calcs. (which of course be not be correct)

 

[desktop987456]

Its almost impossible IMO to measure boiler efficiency using the rads as a accurate indication of load due to flow temperatures, different flowrates through each rad unless perfectly balanced etc.

Can't say they are perfectly balanced but as far as I can tell from copper pipes of rad at flow and return, they seem to be balanced.
I need to play a little with pump speed though. Flow-return delta on boiler increases if I set the pump on c1 setting (return drops to 35.5Degc), but I felt rads were luke warm at that speed; Do I need to balance all the rads at c1 pump setting or should I increase the flow rate on boiler in hope that it will not increase the Return temp to boiler! I haven't tried the proprtional setting on pump as you say balancing rads on PP would be a nighmare.

 

[desktop987456]

At 4.5% condensing and full output your boiler will have a wet gas loss of 12.74*4.5%, 0.573kwh.
The condensate flow will be 0.573/0.628, 0.91 liters

Thanks John for your detailed working on this, I will measure the condensate output again.

I have been very meticulous in terms of gas usage and logging m3 consumed almost every hour during boiler operation but have not been able to control the usage.